We describe the approach for modeling solid-state fluorescence spectra of organic crystalline materials, using the recent implementation of time-dependent density-functional theory within the plane-wave/pseudopotential code CASTEP. The method accuracy is evaluated on a series of organic cocrystals displaying a range of emission wavelengths. In all cases the calculated spectra are in good to excellent agreement with experiment. The ability to precisely model the emission spectra offers novel insight into the role of intermolecular interactions and crystal packing on solid-state luminescence of organic chromophores, allowing the possibility of in silico design of organic luminescent materials.